Single crystal silicon, which is the starting material for most semiconductor electronic component fabrication, is commonly prepared by the so-called Czochralski (“Cz”) method. Using the Cz method, the growth of the crystal is carried out in a crystal-pulling furnace, wherein polycrystalline silicon (“polysilicon”) is charged to a crucible and melted by a heater surrounding the outer surface of the crucible sidewall. A seed crystal is brought into contact with the molten silicon and a single crystal ingot is grown by extraction via a crystal puller. In this pulling process, the quartz glass crucible is subjected for several hours to high temperatures. During that time, the quartz glass crucible raised up through the glasses strain temperature so that before it reaches that temperature it must endure the thermo-mechanical stress and strain. As the temperature is raised above that point, i.e., 1100° C., the viscosity of the glassy silica (SiO2) continues to decrease. At 1421° C. and higher, the glass begins to flow like the high viscosity liquid that it is. Once the polysilicon charge begins to melt, the liquid silicon begins to dissolve the SiO2 crucible, solvating the inner surface and gradually etching silica material off of that inner surface. The crucible must endure these reactions during a typical meltdown and crystal pulling process cycle. The larger the crucible and thus the volume of melt to be received therein, the longer the melting times usually are.
U.S. Pat. No. 5,976,247 discloses a process to enhance the thermal stability of the quartz glass crucible, wherein the crucible is provided with a surface layer of cristobalite. The melting point of cristobalite of about 1720° C. is much higher than the melting temperatures of conventional semiconductor materials (e.g., 1420° C. for a silicon melt) though only somewhat higher than the “melting point” of amorphous silica glass at approximately 1600-1610° C. To generate the cristobalite surface layer, the glassy outer wall of a quartz crucible is treated with a chemical solution containing substances that are conducive to the devitrification of quartz glass into cristobalite (“crystallization promoters”), e.g., alkali metals, alkaline-earth metals, heavy transition metals and barium hydroxide or barium carbonate. When the quartz glass crucible is heated up to a temperature exceeding 1420° C., the surface of the pre-treated crucible wall crystallizes as it transforms to cristobalite, resulting in a quartz glass crucible with better properties for Czochralski crystal growth. The devitrification is persistent and provides an improved degree of control to the dissolution of the crucible surface.
U.S. Patent Publication No. 2005/0178319 discloses providing a quartz glass crucible with a crystallization promoter containing a first alkali-metal component acting as a network breaker and/or network modifier in quartz glass, and a second alkali-metal free component acting as a network former in quartz glass, e.g., an oxide that forms 4 bonds and can form a tetrahedral structure. Examples of these compounds are barium titanate, barium zirconate or a mixture thereof. U.S. Publication No. 2003/0211335 discloses providing a fused quartz article such as a crucible, with enhanced creep resistance by controlled devitrification, specifically by coating the crucible with a colloidal silica slurry doped with metal cations such as barium, strontium, and calcium, to promote nucleation and growth of cristobalite crystals thus prolonging the life of the quartz article.
For the silicon melt to dissolve the crucible, it must reduce the surface of the silica (SiO2) first breaking one of the bonds to form silicon monoxide (SiO), then break up the remaining bond to the network which connected the SiO to the surface of the bulk silica, and finally solvate the individual SiO particles. In U.S. Pat. No. 6,280,522, it is disclosed that quartz crucibles, after extended use, develop small ring like patterns (B) on the surface (A) in contact with the silicon melt, and as time elapses, the patterns change in shape and grow in size as illustrated in FIG. 1. A better description of the phenomena is that surface nuclei form for the growth of crystalline crystobalite (C) species. These small crystal nuclei grow into disk shaped rosette structures (C). The outer perimeter of those disks is oftentimes a light brown or tan color (B), as the outer edge is apparently being reduced and dissolved into the melt faster than the inner portions on the surface of the rosette disk. Sometimes inner portions of the disk come out as tiny specks into the melt (D). Eventually the inner surface region of the rosette disk is dissolved away to expose the glassy surface beneath the crystobalite (E). And, the glassy surface tends to dissolve much more non-uniformly and more quickly than the crystobalite disk. Eventually the inner region expands outward until it reaches the perimeter where the brown ring is located. At this point, the entire crystobalite disk has been eroded away to expose a rough irregular glass surface which can sluff fragments quite easily.
While just the crystalline portion is being eroded away, it comes loose as microscopic fragments which generally dissolve easily. Once the center is dissolved away, it may come loose as larger fragments, since the interface layer between the crystalline portion and the underlying glassy material is dissolved away, undercutting the crystalline material. It should be noted that that interface is naturally weaker since there is a difference in the specific volumes of the crystalline species and the amorphous glassy species. This results in the chemical bonds between the two being strained. It then may contribute particle fragments to the molten silicon. And as the glass surface (E) is eroded or dissolved, it also dissolves non-uniformly and is very likely to let loose particles into the melt which are apt to cause dislocations in the silicon crystal growth and thus reducing the yield. The “brown ring” (B) as illustrated in U.S. Pat. No. 6,280,522 is believed to be silica SiO2 that is reduced to SiO. FIG. 3 is a photograph taken at normal (˜1×) magnification showing the brown rings on the surface of an untreated crucible after one crystal pulling run, with individual rosettes as well as small clusters of rosettes.
Upon close examination of the brown rings in FIG. 3, one observes individual rosettes (as shown in FIG. 2) having low nucleation density with the brown rings being deposits of SiO left on the edge of rosettes. Over time, the rosettes begin to merge with the radial growth increasing until they bump into one another, thus expanding the size of the brown rings of FIG. 1. Eventually, portions of the rosettes begin to flake out, and very often it is the center of the rosettes which begin to flake out as illustrated in FIG. 4, thus causing pitting corrosion.
Today's perfect silicon crystal is grown under conditions where every effort is made to get a crystal structure as perfect as possible. This is done so as to minimize the number of interstitials as well as the number of silicon vacancies. However, even the best attempts to approach thermodynamic/slow growth conditions still incorporate the vacancies. Since interstitials and vacancies cannot be eliminated completely, there is a need to reduce the strain on such a lattice with improved quartz glass crucibles. In one embodiment of the invention, a coating comprising at least a thin film of germanium based species are used for quartz glass crucibles that can be of advantage to the crystal grower.
There is also a need for an improved method to extend the life of quartz crucibles for use in silicon crystal growth. The focus in the prior art has been the reduction in the formation of the rosettes with brown rings. The invention relates to methods to extend the life of quartz crucibles by nucleating other crystalline growth and improving the condition of the rosettes, and contrary to the prior art teaching by increasing the number of rosettes with brown rings formed on quartz crucibles.